Hello! I'm a long time reader of this forum and have only posted one other time. I'm a trained luthier but have been getting into tube amps in the last few years and decided to do it the way I first started with guitars: buy broken things and learn how to fix them. Last year I bought this old Guild amp from the 50s. It isn't part of the Master amp line, and doesn't seem to match any catalogue listings. (I even checked with the guild nerds at the letstalkguild forum) Unsurprisingly there is no schematic or any documentation I can find. So I went about tracing out the schematic, which I will post pics of below, along with pics of the amp chassis. I know this is going to be a very long term project but I want to do it right and try to restore this thing to its full glory. It's clear that someone had been in it at some point because the tremolo channel has sprague orange drops in it. I'm a little worried that the circuit itself has been changed but there's no real way to know I suppose.
Before I start replacing things I want to have the best understanding of the circuit as possible, so I thought I'd ask some questions of this very knowledgeable community. Before diving in, allow me to apologize in advance for my schematic drawing. This is my first attempt at tracing out and drawing a schematic. Not all the component values are listed yet because I'm a little colorblind and have to rely on my partner to confirm the color codes on the components (lol).
Ok first line of questioning: The normal channel seems pretty straightforward but the tremolo channel makes no sense to me. It seems as though the oscillator is in the signal path after the first gain stage, and the speed and intensity controls are far away. This is made extra confusing because there are a couple broken wires (labeled by the question marks on the schematic) but I can't figure out where they would connect. I'll post some close up pictures of the broken wires as well. Does anyone have any particular insights on this? I'm mostly just familiar with fender style tremolos. There's no optocoupler so I would assume this is a bias wiggle of some variety, maybe fluctuating the bias of a preamp tube? But even if that's true I can't figure out what the proper connection should be. This is also the only part of the amp that seems to have been modified in any way so it's quite possible that it isn't stock anymore.
Next, the preamp tube sockets are labeled but not on the power amp chassis. Based on the wiring I feel confident that I properly identified which tube serves which function, but don't know what specific tubes should be used. I have currently assumed the output tubes are 6v6s, that would match the pinout and the era. Both the phase inverter and the gain stage afterward (never seen that before!) Are dual triodes and since the preamp section uses all 6S*7s I'm assuming they are some variety of that. My thought was that down the road after some component replacement, when it's safe to turn it on, I would measure voltages without tubes and use that as a way to cross reference what tube should be used. Is this a good strategy? Does anyone have any other thoughts or tips?
I found info on the acrosound company but could not find info on this specific OT. It says it's ultralinear but it seems like the ultralinear taps are tied together to effectively act as the center tap. But there's one connection to the OT that I can't figure out. There's a wire directly from the OT connected through a resistor to pin 3 of the phase inverter. Assuming this is a 6S*7 dual triode this would be the cathode of the 1st triode. Would this be some form of negative feedback?
I've read many opinions on standby switch functions and placement, and know that the method employed in this amp is not used very often anymore. I like the idea of keeping this as original as possible but want to change out what is necessary for modern safety and reliability standards (i.e. 3 wire power cable, removal of "death cap" etc...). My question is, does having the standby switch on the high voltage secondary center tap of the power transformer present any serious reliability or safety concern? Does anyone have any strong opinions on whether to move or reconfigure the standby switch in this situation?
*more to follow, original post was too long so I have to split it into 2*
schemA by
nick romy, on Flickr
schemB by
nick romy, on Flickr
Topic: Mystery 50's Guild amp, BIG restoration project! (Read 2046 times)